The present invention relates to microelectronic components such as integrated circuits (ICs), printed circuit boards, etc., and more particularly to attachment and electrical connection of microelectronic components to each other or to other circuitry.
Attachment and electrical connection of microelectronic components to each other or other circuitry must meet certain requirements with respect to high mechanical strength, low contact resistance, small size, and other properties. A common technique is to solder the contact pads of different components to each other. Solder attachments have low electrical resistance, can be mechanically strong, and can be quickly formed at low temperatures that do not damage a typical component (e.g. under 450° C.). On the negative side, a strong solder bond requires much solder which can spread sideways when melted and create electrical shorts. Alternative attachment techniques include diffusion bonding, i.e. when the contact pads of different components are bonded together by interdiffusion. However, if the process temperature is low, the diffusion bonding is slow. A still another technique is gluing the contact pads together by conductive or anisotropic adhesive, but the resulting contact resistance can be high.
FIG. 1 illustrates a conventional IC package with semiconductor die 110 attached to a printed circuit board 116 through an interposer (ITP) 120. A die 110 is a semiconductor IC originally manufactured in the same semiconductor wafer (not shown) as some other die and then separated from the wafer. Die 110 are not attached to PCB 116 directly for various reasons. One reason is that the PCB contact pads 116C cannot be positioned as closely together as the die's contact pads 110C due to different fabrication technologies used for the die and the PCB. ITP 120 provides “contact redistribution”: the ITP's top contact pads 120C.T match the die's contact pads 110C, and ITP's bottom contact pads 120C.B match the PCB's contact pads 116C.
Further, in many packages, the PCB is based on non-semiconductor substrates (e.g. ceramic or organic substrates) that have significantly different coefficients of thermal expansion (CTE) than the semiconductor die. The CTE mismatch results in lateral stress on the attachments and may cause the attachments to crack or break. ITP 120 provides a buffer that softens the impact of the CTE mismatch between the die and the PCB. For example, if the die are silicon-based ICs, the ITP substrate 120S may be made of silicon to match the die CTE. As to the ITP-PCB thermal mismatch, this mismatch is less damaging because the bonds between the ITP bottom contact pads 120C.B and the PCB contact pads 116C can be larger (due to their larger spacing) and hence stronger.
In the example of FIG. 1, ITP 120 includes through-holes with conductive vias 130 passing through the ITP substrate 120S. At the top of substrate 120S, the interposer's redistribution layer 140 (RDL) includes conductive lines 140L interconnecting the vias 130 and the interposer's contact pads 120C.T as desired. At the bottom of substrate 120S, the vias 130 terminate at contact pads 120C.B attached to the PCB's contact pads 116C. The PCB's contact pads 116C are interconnected by conductive lines 116L as needed to connect the die to each other and possibly to other circuits (not shown) attached to the PCB. Thus, the top contact pads 120C.T, provided by the RDL, match the die's contact pads 110C; the ITP's bottom contact pads 120C.B match the PCB contact pads 116C; the RDL provides the contact redistribution function, and also provides an extra level of interconnects to augment the PCB's lines 116L. An RDL could also be provided at the bottom of the interposer.
The contact pad attachments are shown at 150. These attachments are solder, but can be of other types.
To ensure reliability of attachments 150 at the top of the interposer, each die's contact pads 110C should all be at the same height; otherwise, if any contact pad 110C is higher than others, the higher contact pad 110C will not reach the corresponding contact pad 120C.T. Likewise, the ITP and PCB contact pads should be at the same height at each side of the ITP. The height uniformity can be disturbed by manufacturing variations and by warpage of the die, the interposer, or the PCB. If connections 150 are solder, the non-uniform height can be partially compensated by making the solder balls sufficiently large, but larger solder balls spread farther sideways to possibly create electrical shorts.
Also, to strengthen the attachment between the microelectronic components, underfill 160 (“UF”) is placed between adjacent components to glue them to each other. A typical underfill material is an organic polymer (e.g. epoxy), possibly with fillers. Commonly used organic polymers have a high CTE compared to silicon. The CTE mismatch undesirably increases warpage which complicates attachment of component assemblies to other circuits and also increases the up-down (vertical) stresses on the contact pad attachments 150 to reduce reliability. The underfill's CTE can be lowered by fillers (additives), but the underfill material has to meet stringent requirements which limit the choice and use of such fillers. Indeed, the underfill should spread between the components without voids. The underfill can be introduced at edges of the components after the components have been attached to each other, and the underfill must flow into the gap between the components to fill the gap within reasonable time and to cure (solidify) without voids. Alternatively, the underfill can be introduced before the attachment of the components to each other, and then the underfill must be reliably pierced by the components' contact pads to establish a low-resistance connection of the contact pads to each other and must cure without voids. These requirements place limitations on the underfill material and reduce the yield of the manufacturing process.